Light-emitting devices are known in which a driver IC for driving light-emitting elements incorporates a temperature compensation circuit so that the light-emission intensity and chromaticity characteristics of many light-emitting elements can be easily maintained constant and uniform despite changes in temperature (for example, refer to Japanese Unexamined Patent Publication No. 2006-135007).
However, the device disclosed in Japanese Unexamined Patent Publication No. 2006-135007 has had the problem that incorporating the temperature compensation circuit in the driver IC makes the circuitry of the driver IC complex, leading to a further increase in cost.
It is known that as long as the amount of light changes within areas close to the chromaticity curve of the black body locus, a natural and balanced light is produced. In view of this, it is known to construct a light-emitting device by combining a plurality of kinds of LEDs whose color temperatures change at different rates according to the applied current, with provisions made so that the output light of the plurality of LEDs as a whole lies close to the chromaticity curve of the black body locus when the light is tuned by adjusting the current applied to the LEDs (for example, refer to Japanese Unexamined Patent Publication No. 2012-113959).
However, in Japanese Unexamined Patent Publication No. 2012-113959, the idea of compensating for changes in color temperature due to changes in device temperature is not disclosed.
In a light-emitting device using an LED, since light is produced by applying current to the LED, the LED is heated due to the application of the current, and its temperature thus changes. Further, the temperature of the LED may also change due to changes in ambient temperature. It is also known that the color temperature of LED output light changes as the temperature of the LED changes.
As a result, when a light-emitting device using such LEDs is applied, for example, as a ceiling lighting apparatus, the chromaticity of the output light of the lighting apparatus changes as the time elapses from the moment it was turned on, and the color appearance becomes different from the color appearance produced immediately after the lighting apparatus was turned on; this can give the user an unnatural feeling. In view of this, attempts have been made to maintain the chromaticity of the LED output light constant by incorporating a temperature compensation circuit as described, for example, in patent document 1.
It is an object of the present invention to provide a light-emitting device having a temperature compensation function implemented with simple circuitry, and a method of temperature compensation in such a light-emitting device.
According to the present invention, there is provided a light-emitting device includes a substrate, a first kind of LED chip which is arranged on the substrate, and which has a characteristic such that Δx and Δy, each representing an amount of displacement on an xy chromaticity diagram, both become negative as temperature rises, and a second kind of LED chip which is arranged on the substrate, and which has a characteristic such that Δx and Δy, each representing an amount of displacement on an xy chromaticity diagram, both are positive as temperature rises.
Preferably, the light-emitting device further includes a current supply terminal for supplying current to the first kind of LED chip and the second kind of LED chip so that chromaticity of light emitted from the light-emitting device stays within a 2-step MacAdam ellipse despite changes in temperature of the light-emitting device.
Preferably, the light-emitting device further includes a diffusing plate.
Preferably, in the light-emitting device, a sealing resin for sealing the first kind of LED chip and a resin frame formed so as to surround the sealing resin are chosen to have the same and/or substantially the same thermal expansion coefficient, and a sealing resin for sealing the second kind of LED chip is chosen to have a thermal expansion coefficient higher than the thermal expansion coefficient of a resin frame formed so as to surround the sealing resin.
According to the present invention, there is also provided a method of chromaticity compensation, includes arranging on the substrate a first kind of LED chip having a characteristic such that Δx and Δy, each representing an amount of displacement on an xy chromaticity diagram, both are negative as temperature rises, arranging on the substrate a second kind of LED chip having a characteristic such that Δx and Δy, each representing an amount of displacement on an xy chromaticity diagram, both are positive as temperature rises, and supplying current to the first kind of LED chip and the second kind of LED chip so that chromaticity of light emitted from the light-emitting device stays within a 2-step MacAdam ellipse despite changes in temperature of the light-emitting device.
Preferably, in the method of chromaticity compensation, a sealing resin for sealing the first kind of LED chip and a resin frame formed so as to surround the sealing resin are chosen to have the same and/or substantially the same thermal expansion coefficient, and a sealing resin for sealing the second kind of LED chip is chosen to have a thermal expansion coefficient lower than the thermal expansion coefficient of a resin frame formed so as to surround the sealing resin.
According to the light-emitting device and the light-emitting device temperature compensation method described above, it is possible to provide a light-emitting device having a temperature compensation function implemented with simple circuitry, and a method of temperature compensation in such a light-emitting device.
Further, according to the light-emitting device and the light-emitting device temperature compensation method described above, since the first kind of LED chip having a characteristic such that Δx and Δy, each representing the amount of displacement on the xy chromaticity diagram, both become negative as temperature rises and the second kind of LED chip having a characteristic such that Δx and Δy, each representing the amount of displacement on the xy chromaticity diagram, both are positive as temperature rises are used in combination to cancel out the temperature shift of the chromaticity, it becomes possible to provide a light-emitting device having a temperature compensation function and a method of temperature compensation without using a special temperature compensation circuit.